The present invention relates to an improved composition for application as a shop primer in the steel industry. More particularly, the present invention relates to a shop primer composition used for coatings that do not interfere with welding operations or give off toxic fumes during these operations.
When bare steel is used for construction purposes, blasting and primer coating have to be carried out in situ, thereby involving higher costs. Therefore, rolled steel is now generally blasted using automatic procedures and immediately coated with a thin shop primer coating prior to being used.
The requirements for shop primers are multiple:
they should be easily sprayable;
they should dry fairly rapidly;
they should provide a good, although temporary, protection against corrosion;
they should provide a primer layer that has good mechanical resistance;
they should provide a primer coating that does not impair weldability;
they should not pose any health hazards during welding;
they should provide a primer coating that is compatible with the further protectons used.
The most common types of welding techniques in the heavy steel construction industry are based on the generation of an electric arc between an electrode and the workpiece, the difference residing in the method used for protecting the weldpool against oxidation. The most important welding techniques include:
1. Manual Metal Arc welding (also called Shielded Metal Arc or Rod welding), wherein the molten seam is protected by the melting of the flux provided around the rod. This welding technique is mainly used in a manual way, although automated horizontal rod welding (also called gravity welding) is known and used. Manual rod welding can be used in all welding positions, but with low welding speed. After welding, the flux remains as slag on top of the weldseam, and it must be removed before any coating operation.
2. Submerged Arc welding (also called under powder welding): this technique is automatic and allows high welding speed, but obviously only for horizontal welds. The powder partially melts and remains on top of the weldseam; the rest is sucked away.
3. MIG/MAG welding (Metal Inert Gas/Metal Active Gas welding), wherein a flow of gas is provided around the electrode core to protect the molten seam. In the case of MIG welding, the gas used is pure argon (or another inert gas). In the case of MAG welding, the gas can be a combination of argon (or another inert gas) with carbon dioxide (sometimes in combination with oxygen). MIG/MAG welding is often preferred because it can be used with automatic welding machines in horizontal welding positions, resulting in higher welding speeds than with other welding techniques. Further, manual MIG/MAG welding with automatic electrode feeding (usually called semi-automatic welding) is also possible; in such case, the welding speed can also be higher than with other manual welding techniques. Still a further advantage of MIG/MAG welding is that the heat-affected zones are smaller than with other electric arc welding methods.
When welding a T-joint, two seams are to be formed on either side. However, when using MIG/MAG welding, the seam solidifies more rapidly. Thus, when the second seam is formed, any gas formed through the combustion of the primer can no longer escape between the parts to be welded. The gas formed is, therefore, likely to cause pore formation in the seam.
It is known to use shop primers based on organic binders, most frequently based on epoxy resins or on polyvinyl butyral reinforced phenolic resins. However, these binders form gases during the welding process, thereby resulting in severe internal (i.e. non visible from the outside) and external porosity of the weldseam. Also, the primer coating alongside the weldseam and on the reverse side is overheated during welding, forming what are called "burn back" zones, and the subsequent paint coatings are likely to perform poorly on the overheated primer. The costly alternative is to remove these primers (i) from the welding area before welding, and also (ii) from the reverse side, preferably before welding else after the welding procedure but prior to application of subsequent coatings. The additional cost of removing the primer, however, reduces the advantage resulting from high welding speed.
It is also known to use shop primers based on inorganic binders, such as silicates. These primers contain a large proportion of zinc to ensure the anticorrosive properties, however, zinc presents important health risks during the welding operations. Further, MIG/MAG welding of steel covered with zinc primers having a high zinc content results in severe spattering during the welding operations. When making vertical weldings, this spattering interferes extensively with the welding process, particularly because the spatters accumulate on the electrode, resulting in short-circuiting and uncontrolability of the weldpool. Since vertical MIG/MAG welding practically has to be carried out semi-automatically, the spattering also creates unacceptable working conditions for the operator.
There is accordingly a need in the art for an improved shop primer composition which would allow good weldability in all positions with low spattering, and reduced health risks without requiring prior removal of the primer coating from the welding area. This need particularly exists when using high speed welding techniques such as in the shipbuilding industry, where welding costs represent as much as one fourth of the building costs.